Fisheries Division, New South Wales Department of Primary Industries, Wollongbar, New South Wales, Australia

Historically, culvert designs have focused primarily on their requirement to move water underneath civil infrastructure, without much concern for the up-stream movement requirements of endemic fish populations. However, it is increasingly recognised by ecologists and infrastructure managers alike, that culverts are a major cause of habitat fragmentation in freshwater ecosystems and a significant contributor in the decline of freshwater fish populations globally. To address this, various culvert remediation designs have been implemented, including the installation of vertical baffles and the provision of naturalistic (rock) substrates. While remediation strategies generally aim to reduce the velocity of water flowing through the structure, there is often resistance to their use because reduced culvert discharge can negatively impact upstream flooding while also resulting in debris clogging and increased culvert maintenance costs. In addition, baffles markedly increase water turbulence which may be detrimental to passage by some fish species or size classes. Here we present some novel remediation designs that exploit the reduced water velocity in boundary layers along the culvert wall to enhance fish passage without significantly compromising discharge capacity. These longitudinal designs produce an expanded reduced velocity zone along the culvert margins that generate minimal turbulence. We show that these novel designs are significantly advantageous to the swimming endurance and traversability for six small-bodied Australian fish species. We also provide data on how and why some culvert baffle designs may impede small-bodied fish passage at high water velocities. This has broad implications for fish community structure where baffles have been implemented to facilitate the passage of large bodied, commercially important fish species.